Corrosion-resistant metallic articles and composition therefor



NOV. 4, 1969 '7 KOHL A 3,476,555

CORROSION-RESISTANT METALLIC ARTICLES AND COMPOSITION THEREFOR Filed March 8, 1966 4 Sheets-$heet l Fl g.1

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CORROSION-RESISTANT METALLIC ARTICLES AND COMPOSITION THEREFOR Filed March 8, 1966 1 4 Sheets-Sheet 2 H. KOHL ETAL CORROSION-RESISTANT METALLIC ARTICLES AND COMPOSITION THEREFOR Filed March 8, 1966 4 Sheets-Sheet 5 Fig.3 STEELJJI .0 .0 w mm m m m m m d 4 il Py il I I f z 7 s v m mlm mm m v v I .I 15 x250 Pofenfial in mV MW m m mzu .mO Ee Hf m a Attorney Nov. 4, 1969- H. KOHL ETAL CORROSION-RESISTANT mmpmc ARTICLES AND COMPOSITION THEREFOR Filed March 8, 1966 4 Sheets-Sheet 4 Fig. 4

STEEL 1y Eu\ E mtmcu 25 4 2 3 A, w J 4 4 4.- m m. m m m m m m m P NNNVID $9, Q I I. III vmwwmlmww m lno e m H Y B i ltt orney United States Patent ABSTRACT OF THE liISCLOSURE An austenitic, readily weldable nonmagnetic steel alloy resistant to stress corrosion cracking and pitting. The alloy consists of up to 0.06% carbon (preferably 0.02 to 0.03% 0.80 to 3.00% silicon (preferably 0.8 to 1.2%), 2.0 to 20.0% manganese (preferably 5.5 to 9.0% 22 to 26% chromium, to 20% nickel (preferably 12 to 15%), 1.2 to 3.0% molybdenum, 0.2 to 1.5% nitrogen (preferably 0.3 to 0.5%), the balance iron with a maximum of 6% tungsten, niobium, tantalum, vanadium, copper or boron.

Our present invention relates to an improved metallic composition having nonmagnetic and good welding characteristics and resistant to corrosion and pitting by corrosive and other influences and, more particularly, to an "improved alloy-steel composition'fhaving a higher yield point and tensile strength than earlier compositions of this general character.

The problem of corrosion of metal bodies has long generated many efforts in metallurgic research and technology to provide relatively high-strength metallic compositions which are resistant to corrosion (e.g. electrolytic erosion) and stress-corrosion pitting or cracking in the presence of corrosive and other detrimental environments. Many efforts along these lines have been undertaken in connection with the metallurgy of steel and other iron alloys since such alloys afford high strength and relatively low cost. One of the major steps forward in this field was the discovery that nickel-chromium steel alloys containing between 16 and 20 weight-percent chromium and 8 to 14 weight-percent nickel were capable of resisting galvanic corrosion and, especially, the formation of fissures, cracks and other dislocations by electrochemical reaction. Typical steel-alloy compositions of this type contained up to 0.08 weight-percent carbon, 1.5 to 4 weight-percent silicon and 0.1 to 0.3 weight-percent nitrogen in addition to the indicated ranges of nickel and chromium. Other nickel'chromium alloys have also been proposed, such alloys containing between 12 and 40% chromiurn, 6 to 40% nickel and at least 1% but no more than 4% by weight copper. The nickel component in steels of the latter type can be partly or completely replaced by less than 4% or more than 6% manganese, or by cobalt.

More recently, austenitic corrosion-resistant chromiummanganese-nitrogen steels have been proposed with a composition of up to 0.06% carbon, up to 2% silicon, 10

,to 20% chromium, 8 to 22% manganese, from 0.05 to a maximum of 0.5% nitrogen and from trace amounts to 2.5% nickel, the balance being iron (all percentages by "ice weight). A composition of this latter type had somewhat improved resistance to stress-corrosion cracking.

Nitrogen-free steels have also been proposed heretofore for incorporation in bodies in which surface pitting (or stress-corrosion cracking) is particularly disadvantageous. Antipitting or pitting resistant steels of this character have compositions of 4 to 20% by weight nickel, 10 to 40% by weight chromium, 1.5 to 3.5% by weight silicon, 1 to 5% by Weight molybdenum and trace amounts to 0.3% by weight carbon. In addition, it has been suggested heretofore to provide steels resistant to pitting in halogencontaining (salt) solutions with a composition of up to 0.07% carbon, 2 to 2.5 silicon, up to 2% manganese, 16.5 to 18.5% chromium, 12 to 14% nickel, and 2 to 2.5% molybdenum, the balance being iron (all percents by weight). Steel alloys consisting of up to 0.06% carbon, 0.35% silicon, 7% manganese, 18% chromium, 9.5% nickel, 1.3% molybdenum, 0.15% niobium, and 0.25% nitrogen or with a composition of up to 0.03% carbon, 0.35% silicon, 19% manganese, 18% chromium, 10% nickel, 1.8% molybdenum and 0.2% nitrogen (the balance being iron with all percents given by weight), have good corrosion resistance in sea water and also are relatively nonmagnetizable so that they have applications for many purposes including the construction of ships bulls and other underwater applications.

While it has been found to be possible heretofore to provide individual steel compositions which have an excellent resistance to corrosion and resist pitting or stresscorrosion cracking in sea water and other detrimental environments, other steel alloys which are nonmagnetizable, and still others Which are weldable, each of these steels has been found to be unsatisfactory in at least one of these respects or to have a relatively low yield point and/or poor weldability. Steel-alloy compositions of this character have, therefore, been cold-worked by conventional forging, rolling, or deformation processes after being formed into an article in order to overcome these disadvantages by increasing the yield point of the body and composition and thus the eifective strength of the metal. It has, however, been found that an increase of the yield point produced by cold-working of a steel-alloy composition is accompanied by a decrease in the magnetic-permeability value of the composition and thus in increasing magnetizability. Moreover, the higher strength obtained by cold-working in this manner is lost in the region of those metallic zones which are heated by welding operations.

It is, therefore, the principal object of the present invention to provide a relatively high-strength steel-alloy composition resistant to corrosion and pitting and characterized by poor or negligible magnetizability and good weldability.

We have now surprisingly discovered that this object and others which will become apparent hereinafter can be attained by the use of a chromium-nic-kel-manganesenitrogen steel alloy of austenitic crystal and grain structure which, while possibly overlapping some of the compositional ranges of known steel alloys, differs markedly therefrom in the relationship of the critical components, and is permanently nonmagnetic and has a strength and yield point far superior to the corrosion-resistance (i.e. resistance to cracking and pitting) of steel alloys provided heretofore. The improved composition of the present invention is an austenitic steel alloy containing up to 0.06% by weight carbon, between 0.8 and 3.00% by weight silicon, 2 to 20% by weight manganese, 22 to 26% by weight chromium, to by weight nickel, 1.20 to 3% by weight molybdenum and 0.20 to 1.50% by weight nitrogen, the balance being constituted by iron and at most 6% by weight of one or more of the following components: tungsten, niobium or tantalum, titanium, vanadium, copper and boron. Both the upper and lower limits of the components mentioned above are critical and it has been found that the system is particularly sensitive to the chromium, molybdenum and nitrogen upper and lower limits. Best results are obtained, however, when the carbon content ranges between 0.02 and 0.03% by weight, the silicon content ranges between 0.80 and 1.20% by weight, the manganese content ranges between 5.50 and 9% by weight, the nickel content ranges between 12 and 15% by weight and the nitrogen content is within the raneg of 0.30 to 0.50% by weight, all percents being of the final alloy composition. These compositions, which according to a principal feature of this invention can be employed as the surface sheet, plate or jacket of all bodies to be exposed to chemical and other environmental pitting elfects and to stress corrosion cracking in sea water, salt solutions (e.g. brines) and other halide-containing solutions, the alloy being equally suitable for use in bodies to be exposed to these effects in their entirety. Suitable objects adapted to be manufactured from this alloy composition include the hull plates of seagoing vessels of all types, caissons to be immersed in sea water, tanks, pipelines, structural-steel shapes for underwater construction, brine-containing vessels and components to be inserted in brine solutions, and the like.

Metallic members composed of the improved compositionare not only resistant to fissure formation and corrosion in general but are also free from or resistant to pitting from other environmental phenomena in solutions similar to sea water and/or containing halide salts and corrosive halogens in general; bodies of this alloy also have a minimum yield point (to a strain of 0.2%) of 45 kp./mm. with a corresponding tensile strength, are substantially nonmagnetizable, and have excellent weldability. The improved steel has an unusually stable austenitic character and has a permeability value which lies substantially below the generally accepted limited of 1.01 gauss/oersted for nonmagnetizable steel. This low permeability does not materially change under the severe cold-deformation stresses which are applied to bodies of, for example, steel plate for use in ship hulls. Moreover, the strength of such steels and their toughness are correspondingly very high. In a quenched state, the yield points (to 0.2% elongation) can increase to values greater than 50 kp./mm. without further treatment. Surprisingly, such steels, in spite of their proportionately high silicon content, evidence excellent weldability and also are not subject to the formation of discontinuities during thermal elongation or shrinkage under rapidly altering thermal conditions.

It has been found that the proportions of silicon, chromium, molybdenum and the relationships of these proportions to one another are highly important to obtain the significant qualities of the improved steel, namely, the durable nonmagnetic character, the high yield point and toughness, the excellent weldability and the resistance to corrosion in sea water and halogen-containing solutions and halogens at low temperatures. The proportions and their relationships of manganese and nitrogen have been found to be especially critical in terms of the weldability of the composition and the increased toughness and strength, both compressive and tensile, of the steel bodies. In fact, the corrosion resistance of the bodies is such that other environmental effects beside those characteristic of galvanic corrosion and pitting are also materially diminished or rendered negligible.

4 EXAMPLE A steel sample containing 0.03% by weight carbon, 1.18% by Weight silicon, 6.86% by weight manganese, 23.31% by weight chromium, 14.30% by weight nickel, 1.48% by weight molybdenum, and 0.421% by weight nitrogen, the balance being iron, has the following characteristic after quenching from the temperature of about 1050 C. in water:

TABLE I Yield point at 0.2% strain kp./mm. 50.3-54.1 Tensile strength kp./mm. 90.4-94.2 Elongation percent 44.0-43.6 Contraction do 71.0-65.0 Notched-bar impact-strength, DVM test mkp./cm. 27.5 Permeability ,u. gauss/oersted 1.003

A steel composition of this type has, as a consequence of the aforedescribed qualities, been found to be especially suitable for use in ship construction where nonmagnetizability, high strength, good weldability and, especially, corrosion resistance in sea water are of exceptional importance. It will be understood, however, that the steels are also suitable for use in machine construction where the aforedescribed characteristics are desirable and can be used with advantage in chemical apparatus and locations at which thesteel may come into contact with chloride-containing solutions similar to sea Water or other halogen-containing media.

In connection with stress-corrosion-fissure formation, it must be observed for the sake of completion that with austenitic steels different types of corrosion are experienced. The so-called classic fissure corrosion arises with all commercially available chromium-nickel steels in hot, highly concentrated chloride solutions and is characterized by the formation of transcrystalline fissures, crevices and other dislocations. .The improved steel of the present invention has been found to be only slightly more resistant to this type of corrosion than the presently available chromium-nickel steels of the character described earlier. However, experiments as to the temperature and concentration dependency of this form of corrosive action has shown that the corrosion process, upon extrapolation, can give rise to such transcrystalline fissure formation in cold sea water only upon the lapse of several hundreds of years.

Of more serious consequence is the discovery that austenitic steels in cold sea water are characterized by an apparently hitherto unknown intercrystalline type of stress corrosion which takes elfect with conventional chromium/ nickel steels in a relatively short time. In fact, chromiummanganese-nitrogen steel of conventional compositions have been found to deteriorate rapidly in cold sea Water as a consequence of corrosion of this latter type whereas the improved steel composition of the present invention is almost completely resistant to this type of corrosion.

The improved corrosion resistance of the improved steel of the present invention in sea water will be readily apparent from a comparison of the current-density/voltage curves of steels of the composition of that of the present invention and conventional corrosion-resistance steel alloys. The polarization characteristics of such steels will be evident from the following description, reference being made to the accompanying drawing in which:

FIGS. 1-3 are polarization graphs of current and current density plotted along the ordinate against potential with respect to a saturated calomel electrode in millivolts plotted as the abscissa for steels I, II and III, respectively, of conventional compositions; and

FIG. 4 is a corresponding plot of the polarization characteristics of a steel IV of the improved composition of this invention.

The graphs of FIGS. 1-4 show the variation of current and current density with potential plotted against a satu- TABLE II C Si Mn Cr Mo Ni Nb N Steel 1, max 0. 07 0. 70 1. 18. 00 2. 50 13. 00 Steel, II, max. 0. 06 0. 35 7. 00 18. 00 1. 30 9. 50 0. 15 0. 25 Steel III, main--. 0. 03 0. 35 19. 00 18. 00 1. 80 10. 00 0. 20 Steel IV 0. 03 1. 18 6. 86 23. 31 1. 48 14. 30 0. 421

The samples were from their open-circuit potential anodically polarized until the current density rose to a value of about 2X10 A/cm. (curve segments a) .and then with the indicated polarization speed were brought to a potential of about l400 mv. (curve segments b). The samples had a surface area exposed to polarization of 0.5 cm. (sample-surface area).

A comparison of the graph of FIG. 4 (steel IV of the present invention) shows that the characteristic curve of this steel has an inflection point substantially higher than that of the other steels, indicating that the steel of the improved composition has a much wider passive range than the samples provided for comparison purposes. This will be readily evident from the graph b which intersects the current density line equivalent to 10' A/cm. at point P for each curve as indicated in the graphs. This intersection point P lies between -l00 and --200 mv. for the steels IIII whereas the intersection point P for the improved steel IV lies at +1000 mv. Since corrosion processes commonly involve a current density between the curves a and b, it is apparent that the steels I-III will undergo corrosive deterioration for example pitting when the potential lies above 100 mv. whereas corrosion of steel IV can occur only with a rise in potential above +1000 mv. Steels with polarization curves of the char acter illustrated in FIG. 4 have not hitherto been known.

Stress-corrosion cracking Sample rods of the improved steel with a test portion of 5 mm. in diameter and 20 mm. in length were placed under a tension of 63 kp./mrn. (about 70% of 0' in aerated boiling sea water prepared in accordance with the German Industrial Standard mentioned above. The test specimen was removed after 2000 hours and found to be completely free from stress corrosion cracking. By contrast, conventional steels after a corresponding period were found under a microscope to have a multiplicity of stress-corrosion formed fissures and crevices.

We claim:

1. An austenitic, readily weldable, substantially nonmagnetizable, steel alloy having a yield point of at least 45 kp./mm. (0.2% elongation) resistant to stress-corrosion cracking in sea-water environments consisting essentially of:

up to 0.06 weight-percent carbon,

0.80 to 3.00 weight-percent silicon,

2.00 to 20.00 weight-percent manganese,

22.00 to 26.00 weight-percent chromium,

10.00 to 20.00 weight-percent nickel,

1.20 to 3.00 weight-percent molybdenum,

0.20 to 0.50 weight-percent nitrogen, and the balance iron with a maximum of 6 percent by weight of tungsten, niobium, tantalum, titanium, vanadium, copper and boron.

2. The alloy defined in claim 1 wherein said carbon is present in a proportion of 0.02 to 0.03 percent by weight of the alloy.

3. The alloy defined in claim 1 wherein said silicon is present in a proportion of 0.80 to 1.20 percent by Weight of the alloy.

4. The alloy defined in claim 1 wherein said manganese is present in a proportion of 5.50 to 9.00 percent by weight of the alloy.

5. The alloy defined in claim 1 wherein said nickel is present in a proportion of 12.00 to 15.00 percent by weight of the alloy.

6. The alloy defined in claim 1 wherein said nitrogen is present in a proportion of 0. 30 to 0.50 percent by weight of the alloy.

7. An austenitic, readily weldable, substantially nonmagnetizable steel alloy having a yield point of at least 45 kp./mm. (0.2% elongation) resistant to stress-corrosion cracking in sea-water environments consisting essentially of:

0.03 weight-percent carbon,

1.18 weight-percent silicon,

6.86 weight-percent manganese,

23.31 weight-percent chromium,

14.30 Weight-percent nickel,

1.48 weight-percent molybdenum,

0.42 weight-percent nitrogen, and

the balance iron.

References Cited UNITED STATES PATENTS 3,152,934 10/1964 Lula 3,235,378 2/1966 Jennings. 3,306,736 2/ 1967 Rundell 75-1285 FOREIGN PATENTS 691,855 8/1964 Canada.

HY LAND BIZOT, Primary Examiner 

